DMOS transistors are double-diffused MOS (metal-oxide semiconductor) field-effect transistors and are used predominantly in power applications. LDMOS transistors, that is, those with lateral current conduction and VDMOS transistors, that is, those with vertical current conduction, belong to this class of transistors. FIGS. 1 and 2 show, in plan and in section, respectively, a portion of an LDMOS transistor with a conventional interdigitated structure. The transistor is formed on a monocrystalline silicon substrate 10 of which a portion defined by a major surface 9 is shown in the drawing. The transistor comprises:
a first region 11 which has n-type conductivity, is a portion of an epitaxial layer, and provides the drain region of the transistor; PA1 a second region 12 which has p-type conductivity, is produced by implantation and subsequent diffusion of doping impurities in the first, or drain region 11, has elongate portions or fingers (only one of which is shown in the drawing), forms a p-n junction with the drain region 11, and provides the body region of the transistor; PA1 a strongly-doped third region 13 which has n-type conductivity (indicated n+ in the drawing), and which provides the source region of the transistor and, with the edges of the body region 12, defines a portion 16 of the body region which provides the channel of the transistor; PA1 a strongly-doped fourth region 14 which has p-type conductivity (indicated p+ in the drawing) and which, in the embodiment shown, is composed of various elemental parts (all indicated 14 in FIG. 1) which extend from the major surface 9 and through the source region 13, establishing contact with the body region 12; PA1 a first electrode 17, for example, of doped polycrystalline silicon, which is insulated from the monocrystalline silicon surface 9 by a thin dielectric layer, preferably of silicon dioxide, overlies the channel 16, and is connected to the gate terminal G of the transistor; PA1 a second electrode 18, for example of aluminium, which is in electrical contact with the third and fourth regions 13 and 14, respectively, on the major surface 9 and thus electrically interconnects the source region 13 and the body region 12 on the surface, and which is connected to a terminal, indicated S in the drawing, of the transistor; and PA1 a third electrode 19, for example, also of aluminium, in electrical contact with the first or drain region 11 on the major surface 9 through a strongly-doped n-type region 21 which favours an ohmic contact between aluminium and silicon, and providing the drain terminal D of the transistor.
An important electrical characteristic of transistors of the type described above is reverse conduction between the source and drain, with the body, source and gate short-circuited. As is known, conduction starts when the voltage V.sub.DS between the drain and source reaches a value such as to bring about reverse conduction in the junction between the body region 12 and the drain region 11. This value, which is indicated as the breakdown voltage BV.sub.DSS, is determined by the physical and geometrical characteristics of the transistor. For the transistor to operate correctly in these conditions, the voltage V.sub.DS must remain constant irrespective of the drain current I.sub.D, as shown in FIG. 3. In certain cases, however, the characteristic V.sub.DS =f(I.sub.D) deviates considerably from the desired characteristic since, as shown in FIG. 4, the voltage V.sub.DS remains substantially constant at the value BV.sub.DSS only for relatively low drain currents and decreases significantly and rapidly for higher currents. This phenomenon, known as "snap-back" considerably limits the field of use of the transistor.
The known behavior described above is attributed to the switching-on of a parasitic bipolar (NPN) transistor which has the source region 13 as an emitter, the body region 12 as a base, and the drain region 11 as a collector. A transistor of this type is shown by broken lines and indicated Tp in FIG. 2. In operation, the current I.sub.D which passes through the body region 12 during the breakdown of the body-drain junction in fact causes a voltage drop within the body region 12 because of its distributed resistance represented by a resistor Rd in series with the base of the transistor Tp. When this voltage drop exceeds the direct conduction threshold of the base-emitter junction, the parasitic transistor starts to conduct so that the voltage V.sub.DS decreases suddenly.
To prevent or attenuate this problem, that is, to shift the switching-on of the parasitic transistor towards higher current values I.sub.D, the doping of the body region 12 is normally increased to reduce the distributed resistance of this region. In certain cases, however, it is impossible or undesirable to increase this doping so as not to forego other characteristics of the transistor or so as not to complicate the normal manufacturing process with additional operations.